[PATCH v2 0/3] vfio/pci: Request resources and map BARs at enable time

Matt Evans
2026-04-23T18:25:47Z

Hi,

This is a replacement for these patches posted to set up the
VFIO barmap in the DMABUF export path, and add serialisation:

https://lore.kernel.org/kvm/20260415181423.1008458-1-mattev@meta.com

Those patches were motivated by the DMABUF export originally missing a
call to vfio_pci_core_setup_barmap() and so possibly exporting a range
for which resources weren't yet requested.

Responses in the thread indicated there wasn't a strong historical
reason to require the bar-mapping to be performed on-demand at BAR
reference time. It's much simpler to move this to
vfio_pci_core_enable(), and that then avoids having to deal with
concurrent lazy requests.

The first patch requests PCI resources and pci_iomap() of the BARs in
vfio_pci_core_enable(), moving this out of
vfio_pci_core_setup_barmap().

Some callers relied on vfio_pci_core_setup_barmap() for its ioremap()
effect, and other callers use it for its resource-acquiring effect.
The function remains in the first patch, containing a cheap check that
both these actions have occurred and keeping the same error behaviour.

The second patch then removes calls to vfio_pci_core_setup_barmap(),
in favour of two small helper functions which make it clearer to
distinguish code that needs just the resource (e.g. mmap) and code
that needs the iomap as well (e.g. the rdwr routines, or virtio).

The third patch adds the resource check to VFIO DMABUF export, which
was previously able to export an unrequested resource. Although patch
1 at first appears to fix this by requesting resources at enable time,
code using the BAR still needs to check the resource really was
acquired.

In future we could reconsider performing all the ioremap()s at startup
time, since a lot of VFIO users will never access BARs in-kernel and
won't need them. Mapping huge BARs will cost some time & memory, and
if necessary this could be reduced by doing the ioremap() portion on
demand.

=== Changes ===

v2:

- Don't fail if resources can't be requested or iomapped, even for
valid BARs, as this would change the userspace-observable error
behaviour. Specifically, if there was an issue with one particular
BAR which happened to never be used, then userspace would never
encounter an error for it. Track iomap and resource-acquisition
status per BAR.

- Break out the checks for resource success from those for iomap
success, in the form of the two new helpers.

- Third patch to add the check to VFIO DMABUF export, because
init-time requests can now fail.

v1:
https://lore.kernel.org/kvm/20260421174143.3883579-1-mattev@meta.com/

Matt Evans (3):
vfio/pci: Set up bar resources and maps in vfio_pci_core_enable()
vfio/pci: Replace vfio_pci_core_setup_barmap() with checks for
resource/map
vfio/pci: Check BAR resources before exporting a DMABUF

drivers/vfio/pci/nvgrace-gpu/main.c | 8 ++--
drivers/vfio/pci/vfio_pci_core.c | 66 +++++++++++++++++++++++------
drivers/vfio/pci/vfio_pci_dmabuf.c | 6 ++-
drivers/vfio/pci/vfio_pci_rdwr.c | 29 +------------
drivers/vfio/pci/virtio/legacy_io.c | 4 +-
include/linux/vfio_pci_core.h | 24 ++++++++++-
6 files changed, 88 insertions(+), 49 deletions(-)

--
2.47.3